The present invention relates to laminated flexibly resilient barrier membranes, and more particularly, to a laminated and flexibly resilient membrane having a multi-layered construction which is useful as a barrier between two different media.
For a further understanding of the present invention, reference can be made to U.S. Patent Application Ser. No. 08/299,287 entitled xe2x80x9cCushioning Device with Improved Flexible Barrier Membranexe2x80x9d which was filed concurrently herewith on Aug. 31, 1994, now U.S. Pat. No. 5,952,065 and is hereby expressly incorporated by reference.
It is known in the prior art that certain barrier membranes which are useful under relatively harsh environmental conditions, e.g., membranes used for pressure accumulators, should exhibit both flexibility and imperviousness. This allows effective transmission of pressures between compartments containing a liquid and compartments containing a gas, respectively, in such accumulators. Unfortunately, there is no single material known which exhibits an acceptable level for both of these properties.
Materials which exhibit acceptable flexibility (such as thermoplastic materials of the polyurethane family) tend to have an unacceptably low level of resistance to gas permeation which results in a loss of the entrapped gas through the material. In contrast, materials which exhibit an acceptable level of resistance to gas permeation tend to have an unacceptably low level of flexibility. Thus, they are not useful in an environment which requires constant flexure.
In an attempt to address the problem of supplying a product which has the characteristics of both flexibility and imperviousness, U.S. Pat. No. 5,036,110 discloses an resilient membrane and a hydro-pneumatic accumulator fitted with that membrane. The first material which provides for the required elasticity is selected from among the thermoplastic polyurethanes, block amide polyethers, flexible polyesters and mixtures of such materials. The second material is grafted onto or embedded into the body of the first material in an effort to provide the required resistance to gas permeation. This second material is noted as being selected from the group consisting of copolymers of ethylene and vinyl alcohol, polyamides, polyvinylidene chlorides and mixtures of such materials. One additional embodiment discloses a film of the second material arranged in a sandwich-like fashion between two layers of the first material. All of the embodiments in U.S. Pat. No. 5,036,110 are manufactured using a bi-material injection press said to be commonly used in the industry of thermoplastics material formation. The perceived problems associated with the bi-material injection molding of the membrane as disclosed in U.S. Pat. No. 5,036,110 include the inability to accurately position and control thicknesses of the various layers within the molded membrane and the inability of the bi-material injection molding process to adequately bond the various materials together to form a unitary wall for the membrane without either creating a graft copolymer, modifying the second material with additional co-monomers, or employing an adhesive or tie-layer.
The primary object of the present invention is to provide a multi-layered membrane which has (1) a desirable level of flexibility (or rigidity); (2) a desirable level of resistance to degradation caused by moisture and (3) an acceptable level of imperviousness to fluids which can be in the form of gases, liquids or both depending mainly on the intended use of the product, while overcoming the problems associated with the prior art. The flexibility and resistance to degradation caused by moisture are generally obtained by using a thermoplastic polyurethane while the imperviousness to media such as liquids and/or gases is generally obtained by using an intermediate layer or a layer generally not exposed to the atmosphere of barrier material such as a copolymer of ethylene and vinyl alcohol, for example.
This object is achieved by using a multi-layer process such as co-extrusion blow molding, or coextrusion of sheet, film, tubing, or profile, for example, which incorporates a separate material flow channel for each material. Typically, first and second extrusion channels for the flexible material (i.e. the thermoplastic polyurethane) are located on either side of the extrusion channel for the barrier material (i.e. the copolymer of ethylene and vinyl alcohol).
The membrane manufactured according to this invention results from laminating the thermoplastic polyurethane and the main barrier material by bringing the selected materials into reactive contact at a temperature of approximately 300xc2x0 F. to 450xc2x0 F. The lamination within the scope of the present invention can be carried out under a variety of plastic forming techniques to create a bond between the two differing materials over substantially the entire intended contact surface of the two differing materials according to the following reaction, when a copolymer of ethylene and vinyl alcohol is employed at least in part as the barrier material: 
According to a first exemplary embodiment of the present invention, the membrane consists of an inner layer of the main barrier material having an average thickness between approximately 10 microns to about 500 microns bonded between a first outer layer and a second outer layer, both outer layers being formed from a material including or consisting essentially of a prepolymeric thermoplastic polyurethane film. The first and second layers of laminate typically have a thickness of at least 0.01 millimeters (mm) for applications such as hydraulic accumulators. It is contemplated that material thicknesses less than 0.01 mm can be employed for other applications such as films used in the food packaging industry. The laminating process of the present invention controls the relative position of the laminates, as well as (in theory) providing for the surface bonding between the laminates. Accordingly, adhesives (or tie-layers as they are often called in the thermoplastic forming industry) are not required or desirable.
According to a second exemplary embodiment of the present invention, the membrane consists of an inner layer comprising a main barrier material having a thickness between approximately 10 microns to about 500 microns which is surface bonded in accordance with the above listed reaction on one side to at least one outer layer of the thermoplastic polyurethane. Again, the reactive contact in the lamination process of the present invention provides for strong surface bonding between the layers thereby eliminating the need for adhesive tie-layers. According to still other exemplary embodiments, membranes consisting of multiple layers (i.e. at least five) including alternating layers of thermoplastic urethanes and main barrier materials are disclosed.
Still other advantages and objects of the present invention will become apparent to those skilled in the art from the subsequent detailed description, appended claims and drawings.
The present invention also relates to the processes and methods for preparing the laminated resilient barrier membranes of the present invention, as well as the membranes so made.